Ryukotsusei is a giant dragonyōkai in the anime and manga series, InuYasha. His name can also be written as Ryukossei since in Japanese, つ (tsu) can mean double consonant as well.
Spoiler warning: Plot or ending details follow.
InuYasha's father first fought him in the older days and sealed him away with one of his fangs. InuYasha stubbornly went to fight Ryukotsusei by himself, but it was Naraku who removed InuYasha's father's seal so that the dragon would destroy InuYasha. Ryukotsusei is ruthless and has a mask that exists is on his forehead through which he speaks. He is able to make large energy blasts from his dragon mouth and could easily have defeated InuYasha with a final blow but instead InuYasha is able to learn and use the Bakuryūha (Backlash Wave in the English dub) and destroy the powerful dragon.
But in 1292 a dragon called Ryukossei came and attacked InuTaishou during his patrol, when he was at the edge of the land claimed by the Kunji family, an insanely rich but compassionate family that hed always liked.
Ryukossei crashed on the beaches of the continent, while InuTaishou was left for dead in the forest where theyd fought.
SesshoMaru and Akina were melting through Ryukosseis armor, leaving a massive scar, when Ryukossei let off a huge blast which hit the top of the mountain and caused a cave-in.
He uses Tetsusaiga's scabbard to fend off the attacks, and Miroku tries to assist using his air rip, but Ryukossei is two powerful for them.
With the new power of the blade he unleashes the Bakuryuha attack, which combines the power of the Scar of the Wind with the attack of an enemy making for an ultra-devestating technique.
Ryukossei dies and Inu-Yasha regains the power of Tetsusaiga and no longer has to worry about his demonic side emerging as long as he possesses the sword.